Ceramic pigments on apatite basis

ABSTRACT

The present invention relates to compounds on apatite basis, having the general formula M 5  (A0 4 ) 3 X wherein X is situated in the hexagonal channels of the apatite structure and includes Cu-atoms, processes for the preparation thereof as well as applications of these compounds. The compounds presented herein are particularly useful as pigments.

The present invention relates to compounds on apatite basis, processesfor the preparation thereof as well as applications of these compounds.The compounds presented herein are particularly useful as pigments.

Ceramic pigments are widely used to impart color and/or opacity toartificial articles and are usually incorporated as fine particles inpaints, plastics, and other materials. Although many chemical compoundsare effectively applied as pigments there is still a need to find newcompounds and mixtures which show bright colors and are relativelycheap, stable towards heating and light, chemically compatible with thematerials, non-toxic and environment friendly.

A compound with approximate composition Sr₅(VO₄)₃(CuO) (1) has beensynthesized and structurally characterized by W. Carrillo-Cabrera, H. G.von Schnering, Z. anorg. Allg. Chem. 1999, 625, 183. However thecompound was colorless and contained copper in oxidation state +1 only.In the same paper an isostructural compound(Sr_(0.9)Ca_(1.1))₅(Cr^(v)O₄)₃(Cu^(I)O) is mentioned, the data for itbeing unpublished. A copper containing strontium vanadate has beendescribed earlier by P. E. Kazin, M. A. Uskova, Yu. D. Tretyakov, M.Jansen, S. Scheurell, E. Kemnitz, Physica C 1998, 301, 185. But only themetal element ratio and the unindexed powder diffraction pattern havebeen reported then. So far, copper has been introduced in the phosphateapatites only in M position. The solid solution(M_(1-x)Cu_(x))₅(PO₄)₃OH, where M=Sr, Ca, as well as the fullysubstituted compound Cu₅(PO₄)₃OH have been reported by M. Pujari, P. N.Patel, J. Solid State Chem. 1989, 83, 100. Besides, the phosphorSr₅(PO₄)₃Cl doped by small amount of Cu as activator is known (Hunt,McKeag, J. Electrochem. Soc. 1959, 106, 1032).

Therefore, it was an object of the invention to provide new compounds onapatite basis, especially compounds which are brightly colored.

According to the invention this object is achieved by providing acompound having the general formula (I)M₅(AO₄)₃X,wherein the group M₅(AO₄)₃ forms an apatite structure and X is situatedin the hexagonal channels of the apatite structure and includesCu-atoms, with the proviso that the compound is not Sr₅(VO₄)₃(CuO),Sr₅(VO₄)₃(Cu_(0.894)O_(0.952)), or (Sr_(0.9)Ca_(0.1))₅(Cr^(v)O₄)₃(CuO).

In particular, the invention relates to new chemical compounds,processes of their preparations and applications. The compounds haveapatite based structures with general composition M₅(AO₄)₃X where M andA are different individual chemical elements or mixtures of elementsappropriate to form the apatite structure.

In a preferred embodiment A represents P, V or a mixture thereof and Mrepresents Ba, Sr, Ca or a mixture thereof. X represents different atomsin different quantity, situated in the hexagonal channels. Essentialfeature of the compounds is the presence of Cu, in particular, of Cuions in the channels. Most preferably, the Cu ions form O—Cu—O linearunits.

The compounds of the invention, in particular, compounds having Cu²⁺ inan unusual two-fold coordination, are brightly colored from dark-bluethrough blue-violet to red-violet. The color shade can be regulated byusing different M and A elements, the brightness can be increased ordecreased by oxidizing or reducing the copper ions via annealing inatmosphere with different partial oxygen pressure. The compounds arestable in ambient conditions and toward heating above 1000° C. in air.The compounds with certain M, A and X are relatively cheap, non-toxicand environment friendly. The compounds are preferably applied aspigments for plastics, paints, cements and plasters.

The group X preferably has a charge of −1 and formally represents acertain fraction of Cu²⁺, and/or Cu⁺, and O²⁻ ions mixed with anionssuch as OH⁻, F⁻, Cl⁻, Br⁻. Most preferably, X representsCu_(x)O_(y)H_(z), wherein 0<x≦0.85, 0≦z<1 and 0.5<y≦1. More preferably,X represents Cu_(x)O_(y)H_(z), wherein 0.1≦x≦0.6, in particular,0.2≦x≦0.5.

Several samples -of compounds of the invention were characterized byX-ray single crystal and powder diffraction, ICP-OES analysis, scanningelectron microscopy with EDX analysis, IR and NMR spectroscopy, magneticmeasurements, UV-VIS spectrometry (diffuse reflectance spectra).

In preferred embodiments the main new features of the compounds claimedin the invention are: (i) copper(II) exists in the hexagonal channels ofthe apatite structure and provides the bright color of the samples;or/and (ii) copper ions are present in the hexagonal channels ofphosphate apatites and other compounds with apatite structure; or/and(iii) copper atoms as oxocuprate ions form a continuos solid solution inthe hexagonal channels, optionally together with other anions.Essentially unexpected is the incorporation of copper-oxygen units inthe place of hydroxyls in the well known phosphate apatites, as well asthe presence of divalent copper in the channels in twofold coordinationby oxygen atoms. According to (ii) copper ions in an oxidation state of+1, +2 or both, are in the channels of apatites. Examples of apatitesare phosphate apatites as well as vanadate apatites. By selectingappropriate elements for A also apatite structures based on SiO₄ ²⁻,SiO₄ ⁴⁻ and AsO₄ ³⁻ can be prepared.

It has been reported that in channels of the particular apatiteSr₅(VO₄)₃CuO long, ideally infinite, linear chains of Cu_(n)O_(n+1)^((n+2)−) are present. According to (iii) above Cu—O⁻ units can becontinuously substituted by anions present in apatites such as OH⁻, F⁻or Cl⁻ so that the long [CuO]_(n) ^(n−) chains are broken into shortentities such as monomeric O—Cu—O³⁻, O—Cu—O²⁻, HO—Cu—O²⁻ or oligomericO—(Cu—O—)_(n)—Cu—O, wherein n is an integer from 1 to 10, preferablyfrom 1 to 5, and more preferably from 1 to 3.

Compounds containing Cu²⁺ and, in particular, compounds containing Cu²⁺as well as Cu⁺ are brightly colored. They can be used, in particular, aspigments, e.g. as ceramic pigments.

Further, the compounds of the invention, in particular, compounds havingonly Cu⁺ and no Cu²⁺ in group X preferably can be used as intermediates.From these compounds colored substances can be obtained, e.g. byoxidation. To impart color or to enhance color it is often sufficient tooxidize only a small part of Cu(I) to Cu(II).

The compounds according to the invention can easily be prepared bymixing compounds comprising the elements M, A and X and thermallytreating the mixture in a range of from 200 to 1700° C. to yield acompound of general formula (I). The starting compounds preferably areemployed in a ratio close to the desired stoichiometric ratio. Thethermal treatment preferably takes place at 400 to 1500° C., morepreferably from 700 to 1400° C. The thermal treatment can be performedfor 0.01 to 60 h, more preferably from 0.1 to 30 h, and most preferablyfrom 1 to 10 h. To enhance the yield of the desired compounds thethermal treatment can be performed with intermediate regrinding.Depending on the amount of Cu²⁺ desired, the thermal treatment can beperformed in an oxygen-containing atmosphere such as air or oxygen or inan oxygen-free atmosphere such as under argon, nitrogen or anotherprotective gas. To enhance the amount of Cu²⁺ present in the hexagonalchannels of the apatite, an additional step can be performed comprisinga thermal treatment of the compound in oxygen, inert gas atmosphere orvacuum at 500 to 900° C., preferably from 600 to 800° C. for 0.5 to 24h, preferably from 2 to 12 h.

In a most preferred embodiment the process according to the inventioncomprises the steps:

-   (i) mixing of carbonates of M, (NH₄)H₂PO₄ and Cu compounds,-   (ii) thermal treatment of this mixture in solid state in air at 600    to 850° C. for 1 to 5 h,-   (iii) regrinding,-   (iv) thermal treatment at 1100 to 1400° C. for about 1 to 24 h,-   (v) cooling, and-   (vi) regrinding.

The compounds M₅(AO₄)₃X preferably can be prepared from mixtures ofsalts and oxides containing the required components in close tostoichiometric ratio by solid state reaction at about 700-1400° C., orby melting and solidification at about 1000-1700° C. The samplesprepared in air contain simultaneously Cu³⁰ and Cu²⁺ ions. Depending onthe copper content and thermal treatment conditions separate linearO—Cu—O or/and condensed (−)O—Cu—O—Cu—O(−) units form in the hexagonalchannels. The copper can be further oxidized by annealing in oxygenatmosphere or reduced by annealing in argon atmosphere or vacuum with aCu₂O—CuO getter.

In particular, samples containing Cu²⁺ exhibit an unexpectedly brightcolor for copper ions. The absorption spectra show overlapping bands inthe visible region. They can be related to d-d electron transitions inlinear O—Cu(II)-O units. The latter appear to be the first example oftwofold coordinated divalent copper in the solid state. The color ischanging from dark-blue for Ba₅(PO₄)₃Cu_(x)OH_(z) through blue-violetfor Sr₅(PO₄)₃Cu_(x)OH_(z) till red-violet for Ca₅(PO₄)₃Cu_(x)OH_(z). Thebrightest color is attained for x about 0.1-0.4. The color intensity isfurther increased by annealing the samples in oxygen at 600-800° C.

The compounds of the invention preferably are stable in air at ambientconditions, insoluble in water, and resistant to heating up to 1000° C.Compounds with M=Sr, Ca, A=P, and X=CU_(x)OH_(z) are non-toxic. Amongthem, the compounds with M=Ca are very cheap and environment friendly,as they represent simply hydroxylapatite modified by inserting smallamounts of copper into the hexagonal channels. Further, the compounds ofthe invention are stable in alkaline media due to the basic nature ofhydroxylapatites.

The invention further relates to a pigment comprising a compound of theinvention, in particular, a compound of general formula (I), wherein Xcomprises Cu²⁺, more preferably, wherein X comprises Cu²⁺ as well asCu⁺.

The compounds of the invention can be used as intermediates forpreparation of colors as well as colored materials themeselves.Therefore, they can be used, e.g. as pigments, paint or as coloringadditives, e.g. in cements or plasters.

The following Figures and Examples illustrate this invention.

FIG. 1 shows diffuse reflectance spectra of the samples of Example 1.Sr₅(PO₄)₃Cu_(x)OH_(y); as prepared: A1, x=0.1, A3, x=0.3, A5, x=0.5;annealed in oxygen at 800° C.: D1, x=0.1, D3, x=0.3.

FIG. 2 shows diffuse reflectance spectra of the samples of Example 4 andExample 5, M₅(PO₄)₃Cu_(0.3)OH_(y).

EXAMPLE 1 Preparation of Sr₅(PO₄)₃Cu_(x)OH_(y), about 1-5 g

SrCO₃, NH₄H₂PO₄ and CuO (all of 99.99%) were thoroughly ground and mixedin an agate mortar in a 5.05:3: x molar ratio, where x=0.01, 0.1, 0.2,0.3, 0.4, 0.5, 0.6. The mixtures were heated stepwise at 600 and 850° C.for 32 h with intermediate regrindings. The powders were pressed inpellets, annealed in air at 1100° C. for 24 hours and air-quenched. Thepellets were ground in an agate mortar to obtain fine powders.

X-ray diffraction patterns correspond to almost single phase apatite(about 97% for x=0.01 and >99% for the other x). Rietveld structurerefinement have confirmed the copper to be situated in the hexagonalchannels. Infrared spectra testify presence of OH groups with very weakhydrogen bonds. The samples are blue-violet in color, deepening withincreasing x. The diffuse reflectance spectra are shown in FIG. 1. Twooverlapping lines are observed in the visible region. The samples withx=0.1 and 0.3 were further annealed in oxygen flow at 800° C. for 2hours. By this treatment, the intensity of the absorption lines has beenincreased.

EXAMPLE 2 Preparation of Sr₅(PO₄)₃Cu_(0.3)H_(y), 5 kg

20 moles rough crystalline NH₄H₂PO₄ and 33.67 moles SrCO₃ werethoroughly ground and mixed for 1 hour in a mill. A water solution of 2moles Cu(NO₃)₂*2.5H₂O was added and the components were mixed for 1 houragain. The mixture was annealed stepwise at 600° C. and 1200° C. fortotal time of about 36 hours with an intermediate regrinding and airquenched.

The powder obtained is bright blue-violet in color.

EXAMPLE 3 Preparation of Sr₅(PO₄)₃Cu_(1/3)O_(2/3), about 1 g

The sample (about 0.5 g) with x=1 prepared as in Example 1 wasarc-melted on a copper plate in air at about 1700° C. for about 1-2minutes and air quenched. The sample is blue-violet in color andcontains>98% of apatite phase.

EXAMPLE 4 Preparation of M₅(PO₄)₃Cu_(0.3)OH_(y), where M is Ca or Ba,about 1-5 g

MCO₃, NH₄H₂PO₄ and CuO (all of 99.99%) were thoroughly ground and mixedin an agate mortar in a 5.05:3:0.3 molar ratio. The mixtures wereannealed stepwise at 600, 850° C. and 1100° C. for total time of about60-80 hours with intermediate regrindings and air quenched.

The sample with Ca is red-violet in color, and the one with Ba isdark-blue. The corresponding diffuse reflectance spectra are shown inFIG. 2.

EXAMPLE 5 Preparation of Sr_(2.5)M′_(2.5)(PO₄)₃Cu_(0.3)OH_(y)

The same preparation procedure as in Example 4 but instead of MCO₃ anequimolar mixture of SrCO₃ and M′CO₃ was used.

The samples have intermediate colors between these of the samples ofExample 2 and Example 4. The diffuse reflectance spectrum of the samplewith M=Ca is shown in FIG. 2.

EXAMPLE 6 Preparation of M₅(VO₄)₃Cu_(0.3)O_(1-z)H_(y), where M is Sr orCa

The same preparation procedure as in Example 4, but 1/2 V₂O₅ was takeninstead of NH₄H₂PO₄.

As obtained apatite phases have gray-blue-violet (M=Sr) orlight-gray-green (M=Ca) colors.

EXAMPLE 7 Preparation of M₅(PO₄)₃Cu_(x)(O,X)_(y)H_(z), where M is Ca orSr, X is F or Cl

The same preparation procedure as in Example 4, but 0.2-0.7 mol of NH₄Xwas added to the initial mixture.

The samples show colors ranging from pink to blue-violet.

EXAMPLE 8 Preparation of Sr₅(PO₄)₃Cu_(0.3)OH_(y), ca. 250 g

1 mole NH₄H₂PO₄ and 1.68 mole SrCO₃ were thoroughly ground and mixed for1 hour in a mill. A water solution of 0.1 mole Cu(NO₃)₂*2.5H₂O was addedand the components were mixed for 1 hour again. The mixture was annealedstepwise at 600° C. and 1100° C. for total time of about 24 hours in arotary kiln (Al₂O₃ tube, d_(i)=12 cm, 1=50 cm, rotation speed 1 min⁻¹).Thereafter the furnace tube was inclined so that by rotation the powdercame down and fell into a cold container. After cooling, the powdercollected in the container was reground.

The powder obtained is bright blue-violet in color.

EXAMPLE 9 Preparation of Sr₅(PO₄)₃Cu_(0.3)OH_(y), 1-10 g

SrCO₃, NH₄H₂PO₄ and Cu(NO₃)₂*2.5H₂O (all of 99% purity) were thoroughlyground and mixed in an agate mortar in a 5.05:1:0.3 molar ratio. Themixture was heated at 600° C. for 6 hours. Then the reground mixture wasplaced in a tube furnace equipped with different operating atmospheremodes and heated in an air-flow at 1100° C. for 3 hours. Thereafter theworking temperature was decreased to 1000° C., the operating atmospherewas changed to dry-air, and the mixture was further cooled down to roomtemperature. The rate of cooling steps in both cases was programmed tobe 200° C./h.

The powder is bright blue-violet.

EXAMPLE 10 Preparation of Sr₅(PO₄)₃Cu_(0.3)OH_(y), 1-10 g

The same preparation procedure as in Example 9, but after changingatmosphere to dry-air the furnace was switched off.

The sample is bright blue-violet.

EXAMPLE 11 Preparation of Sr₅(PO₄)₃Cu_(0.3)OH_(y), about 1-10 g

The same preparation procedure as in Example 9, but with an extra post-annealing of the mixture in an oxygen flow at 600° C. for 2 hours.

The color of the sample is the same as in Example 9, but brighter.

EXAMPLE 12 Preparation of Sr₅(PO₄)₃Cu_(0.3)OH_(y), about 1-10 g

The same preparation procedure as in Example 9, but the cooling of themixture in dry-air was performed only down to 600° C., afterwards theoperating atmosphere was changed to oxygen, the mixture was heated at600° C. for 2 hours, and further cooled down to room temperature.

The color of the sample is the same as in example 9, but brighter.

1. A compound having the general formula (I)M₅(AO₄)₃X, wherein the group M₅(AO₄)₃ forms an apatite structure and Xis situated in the hexagonal channels of the apatite structure andincludes Cu-atoms, with the proviso that the compound is notSr₅(VO₄)₃(CuO),Sr₅(VO₄)₃(Cu_(0.894)O_(0.952)), or(Sr_(0.9)Ca_(0.1))₅(Cr^(v)O₄)₃(CuO).
 2. The compound according to claim1, wherein A represents P, V or a mixture thereof and M represents Ba,Sr, Ca or a mixture thereof.
 3. The compound of claim 1, wherein Xrepresents a mixture of Cu²⁺, Cu⁺, O²⁻, OH⁻, F⁻, Cl⁻, Br⁻ and/or 1⁻. 4.The compound of claim 1, wherein X comprises copper ions.
 5. Thecompound according to claim 1, wherein X comprises Cu²⁺.
 6. The compoundaccording to claim 1, wherein linear units O—Cu—O are present in thehexagonal channels of the apatite structure.
 7. The compound accordingto claim 1 wherein X represents Cu_(x)O_(y)H_(z), wherein O<x≦0.85,O≦z<1 and 0.5<y≦1.
 8. The compound according to claim 7, wherein0.1≦x≦0.6.
 9. The compound according to claim 1, wherein A represents P.10. A process for preparing a compound according to claim 1 comprisingthe steps: (i) mixing of compounds comprising the elements M, A and X,(ii) thermal treatment of the mixture in the range of 200 to 1700° C. toyield a compound of the general formula (I).
 11. The process accordingto claim 10, wherein the thermal treatment is performed for 0.01 to 60hours.
 12. The process according to claim 10, wherein the thermaltreatment is performed when intermediate regrinding.
 13. The processaccording to claim 10, wherein the thermal treatment of the mixture isperformed in air, argon or oxygen.
 14. The process according to claim10, further comprising the step (iii) thermal treatment of the compoundobtained in step (ii) in oxygen, inert gas atmosphere or vacuum at 500to 900° C. for 0.5 to 24 hours.
 15. The process for preparing a compoundof claim 1 comprising the steps (i) mixing of carbonates of M,(NH₄)H₂PO₄ and Cu-compounds, (ii) thermal treatment of this mixture insolid state in air at 600 to 850° C. for 1 to 5 hours, (iii) regrinding,(iv) thermal treatment at 1100 to 1400° C. for about 1 to 24 hours, (v)cooling and (vi) regrinding.
 16. Pigment comprising a compound ofclaim
 1. 17. Pigment according to claim 15, wherein X in the compound ofgeneral formula (I) comprises Cu²⁺.
 18. Use of a compound according toclaim 1 as pigment, paint or as coloring additive in cements orplasters.
 19. Pigment comprising a compound of claim
 10. 20. Pigmentcomprising a compound of claim 15.